\begin{tikzpicture}
\begin{axis}[ybar, ylabel=Elapsed Time (sec), ymin=0, axis y line*=left,
  xlabel=Number of Molecules,
  xmin=0, xmax=7, xtick={1,2,3,4,5,6},
  xticklabels={$2^3$, $4^3$, $6^3$, $8^3$, $10^3$, $16^3$},
  legend pos=north west, legend columns=2]
\addplot coordinates
{(1, 0.1738) (2, 3.0888) (3, 9.93) (4, 28.9722) (5, 39.4486) (6, 55.2144)};
\addplot coordinates
{(1, 0.094) (2, 1.6366) (3, 5.385) (4, 16.9468) (5, 21.9326) (6, 31.3942)};
\legend{Chapel, C}
\end{axis}

\begin{axis}[xmin=0, xmax=7, ymin=0, ymax=1,
axis y line*=right, axis x line=none, ylabel=Speedup]
\addplot[mark=*, color=teal, update limits=false] coordinates
{(1, 0.54085) (2, 0.52984) (3, 0.54229) (4, 0.58493) (5, 0.55598) (6, 0.56859)};
\end{axis}
\end{tikzpicture}
